An x-ray tube outputs radiation across a wide range of energy bands, the distribution of the energy being defined by the accelerating voltage applied to the tube. When x-rays impact a material, they are absorbed as they pass through. X-rays of different energies are absorbed differently which means that the initial x-ray intensity profile changes. Different materials cause a distinctive change in shape of the x-ray intensity spectra and thus if the spectra can be recorded with sufficient accuracy, it is possible to predict the material that the x-rays have passed through.
While the mass absorption coefficient depends upon both the material type and also the energy of the incident photons, the mass absorption coefficient is independent of material thickness and density. Hence, faced with a resultant spectrum, and knowing the starting spectrum, it is possible to deduce the mass absorption coefficient values and hence the material type the x-rays have passed through.
The detection of x-rays falls into two categories. The first is direct detection, where the energy of an x-ray photon impinging upon a particular material can be directly detected.
Direct detection allows materials to be identified in the manner described above.
For example, X-ray detectors such as LiNbO3 or Ge can directly detect the energy of the incident x-rays and thus are able to output the energy spectrum of the x-ray allowing a determination of the material type.
This technology is described in a number of published patent applications. For example:
The international patent application published under number WO2008/142446 describes energy dispersive x-ray absorption spectroscopy in scanning transmission mode involving the calculation of the intensity ratios between successive frequency bands;
The international patent application published under number WO2009/125211 describes an imaging apparatus and method;
The international patent application published under number WO2009/130492 describes the determination of composition liquids; and
The international patent application published under number WO2010/136790 describes a method for the identification of materials in a container.
Whilst the techniques set out in the patent applications mentioned above are effective, the detectors themselves present limitations. The direct x-ray detectors needed are very expensive and the electronics required to operate them is relatively unsophisticated. This leads to errors arising from phenomena such as dark noise, pulse pile-up and energy band fluctuations. In addition, it is difficult to achieve detectors with more than 1024 pixels, which restricts detectors to simple area or pure line-scan detectors. These detectors restrict the scope of the available applications that direct detector technology can be applied to.
The second category of x-ray detection is known as indirect detection, where x-ray photons are first converted into a visible light signal by a scintillator, and then the visible light is detected.
Traditional thinking is that during conversion of x-rays to visible light in a scintillator any energy information that the x-ray photon originally had is destroyed, the visible light output of a scintillator representing only the incident photon density. Hence, the use of indirect conversion has traditionally been limited to pure imaging techniques, whereas direct conversion has been deployed in relation to materials identification.
Dual-energy line scanners are a development of x-ray detectors utilizing scintillators. Dual-energy line scanners utilize two sets of detectors, each including a scintillator, the two sets of detectors being spaced apart but axially aligned, with the material under analysis situated between the x-ray source and the first of the two scintillators. The scintillators are specified such that a part of the x-ray spectrum emitted by the source and passing through the material is absorbed by the first scintillator, so that the x-ray energy spectrum impinging on the second scintillator is not identical to that impinging on the first scintillator, even when a material for analysis is not present. Hence, the photon emission by each scintillator is different. A material under analysis affects the difference in photon emission by each scintillator. Materials may be identified by looking at the ratio between the photon emission by the respective first and second scintillators.
Another type of dual energy detector is described in EP1063538. Instead of the x-rays passing through the specimen and two axially aligned detectors, a linear array scintillator system is provided comprising alternating scintillator thicknesses, one of the thicknesses of scintillator material providing a signal corresponding to the passage of x-rays through a material of relatively low molecular weight and the other thickness of scintillator material providing a signal corresponding to the passage of x-rays through a material of a relatively high molecular weight. The linear array described is for use with a thin collimated curtain of x-rays that pass through an object under test and impinge upon the linear array scintillator. According to EP1063538, the linear array scintillator can produce a stereoscopic image and discriminate between materials of high and low molecular weight more easily than dual energy detectors of the prior art.
Dual-energy line scanners are effective in identifying materials having a high or a low molecular weights. However, they are not particularly effective in identifying materials having a molecular weight falling between these extremes or in distinguishing the difference in materials with a very similar molecular weight.
The ability to detect a wide range of materials using indirect detection utilizing a scintillator would be particularly advantageous. This is because, in comparison to the equipment used for the direct detection of x-rays, equipment for indirect detection utilizing a scintillator is comparatively cheap, and also sophisticated electronics have been developed. Further, the resolution available with direct detectors limits the number of pixels into which an image may be divided, at present to about 1000, whereas those using such detectors would like to increase the number of pixels into which an image is divided significantly.
Whilst dual-energy detectors have some material identification capability, that capability is limited.
It would therefore be desirable to provide an indirect x-ray detector including a scintillator, capable of identifying materials.